Electronic Troubleshooting

Rotation VIII Troubleshooting
(Getting your board to boot)

The following summary is intended to be a troubleshooting guide for the electronics in Midway’s Rotation VIII.

As you have probably already noticed, the circuit boards used in this machine are not used (to my knowledge anyway) in any other machine. As well, there are virtually no self-diagnostics available to help resolve machine electronic faults.

This guide assumes you have a reasonable understanding of electronic repair, have good soldering skills, can remove and replace components including DIP components, have basic electronic testing equipment and can read a schematic. An oscilloscope would be helpful, but a reasonable logic probe may still work.
Main Board

A little background on the main control board first. The main board is based on a Z-80 processor. The Z-80 processor uses separate pins for the address buss (16 lines) and the data buss (8 lines). This address and data buss all pass through to the daughter board which holds the 2716 style eproms and the NV memory. The daughter board also generates the RD (data read) signal used by a number of components on the main board.

Now, to add another level of complexity, the designer, I’m sure in his or her infinite wisdom, chose to use several additional components originally designed for the 8080 and Z-85 processors, and here is where the rub comes in. These two possessors used multiplexed data and address lines, in other works, the lower 8 address bits share the same lines as the 8 data bits. That means in addition to the 16 address lines and 8 data lines on the Z-80, you have another 8 other lines that can have data or address.

So, unless everything on the two separate address and data busses including the daughter board work, the main board will not boot. The main board is smart enough to send itself a reset pulse if it does not boot within a certain amount of time. Monitoring that reset line will give the trouble shooter an indication that the board is trying to boot, but is not successful.

OK, have I scared you yet?

Having repaired a number of main boards, there are a couple of common faults that may be encountered. Before starting any diagnosis, I am assuming that all voltages have been checked, connectors are solid and all boards are in place. I’m also assuming the main board does not show corrosion damage.

Situation 1) Turn on machine and nothing happens, only GI lights come on, no table movement and no controlled lamps are lit.

First check pin 2 of J8. A solid high indicates the reset is not active and is normal. A machine booting normally will have a steady voltage of around 4.8 volts on this pin. This condition is highly unlikely in a non-booting board and, if present, is probably caused by a faulty reset circuit or loss of the clock signal. A solid low signal is also a reset fault and will not allow the processor to operate. Pin 10 of A1 should have clock. If it doesn’t, start trouble shooting the clock circuit including B1. If the clock signal is present at A1, check the reset circuit in chips A1, A2 and A3 for faults.

If you have a continuous series of pulses on J8, pin2, you could have a fault in the daughter board, and/or either of the buss drivers F3 and H3.

Examine the daughter board carefully for any signs of corrosion. I have had nothing but grief with this board. Even the slightest bit of corrosion seems to cause the daughter board to fail. If you have access to a known good daughter board, or have access to my replacement daughter board, try it to see if the main board will boot.

Replace the buss drivers at F3 and H3 (8304 and 81LS95). Failure of these chips seems quite common and probably results from a failure on the daughter board. Even with an oscilloscope, I have never had much luck in determining which of the two chips is bad, so as a matter of practice, I replace both.

With a good chip in F3 and H3, hopefully you will now be in situation 2 below or better yet the main board boots.

Situation 2) Power on and random switched lights, solenoids and/or table movements and no start-up tune.

In most cases, this will happen with a bad daughter board and/or faulty solder joins of the connectors to the daughter board. Have I mentioned how much I hate the original daughter board?

Reflow solder on daughter board and main board connectors.

Check presence of all three voltages on the daughter board.

Slightly wiggle and twist the daughter board as the machine is turned on, anything different?

Reseat the three TMS2716 chips.

Still nothing? You may be in a situation needing further diagnosis. Time for us to talk.

Situation 3) Power on, board boots and plays a tune, but as it boots, you get random solenoid firing.

Upgrade the reset circuit as I outlined here.

Motor Driver Board

This board will commonly fail, usually when anything gets in the way of the rotating table. Wires, cables, almost anything can cause it and the fuse rarely protects it.
The good news is that this board is easily repaired. Usually what happens when one of the driver transistors fail is the table will turn in one direction only, cannot correct for an overshoot and will not start a game. Almost always, the failure will be one of the Darlington pair transistors, TIP 110 or TIP 115. All four are easy to replace and are commonly available.

Occasionally, if the Darlington transistor drivers are good and the table does not move, the problem may be in the large 3.3 ohm resistor. Check it for continuity. Rarely the LM3900 goes bad, but it can happen.